There is a fundamental gap in our understanding of the immune reconstitution inflammatory syndrome (IRIS), which occurs frequently in HIV+ patients with progressive multifocal leukoencephalopathy (PML) treated with antiretroviral medications. This poses a difficult diagnostic and therapeutic conundrum, since neurological deterioration associated with inflammation in PML-IRIS cannot be differentiated from the demyelination occurring with the natural evolution of PML. In addition, a growing number of PML patients are developing seizures, thought to originate in the cerebral cortex. How PML, a disease affecting predominantly the CNS white mater triggers seizures is unknown. Our long term goal is to understand the pathogenic mechanisms of PML-IRIS, and the cause of epileptogenesis in PML and its association with IRIS. Our objective is to establish precise criteria to diagnose and track IRIS and predict the development of seizures in PML, which can then be used for preventive and therapeutic purposes. We have shown that the T-cell response to the causative agent of PML, JC virus (JCV), was associated with PML survival. Using proton magnetic resonance spectroscopy (1H-MRS) and arterial spin labeling (ASL) MRI, we have defined differences in the metabolism and perfusion of PML lesions with and without IRIS. Finally, we have identified a novel MRI marker associated with seizures and IRIS. This hyperintense cortical signal (HCS) can be seen on pre- contrast T1-weighted images in cortical gray matter adjacent to PML lesions. We hypothesize that hyperperfusion seen in PML progressors delineates virologically active lesions and is triggered by local production of nitric oxide (NO). We postulate that HCS is caused by an infiltrate of macrophages and astrogliosis in deep layers of the cerebral cortex affected by JCV demyelination, and constitutes the nidus of epileptogenesis in PML. We propose that IRIS is caused by an imbalance between Th1, Th2 and Th17 immune responses. The rationale for the proposed research is that a combination of immunological, neuroradiological and electrophysiological parameters will predict the development of IRIS and seizures in PML, and directly help in the management of these challenging patients. To test these hypotheses, we will pursue the following set of Specific Aims: 1)Characterize immunological and radiological determinants of inflammation and outcome in PML patients with and without IRIS. We will study JCV-specific T-cell responses, MRI, 1H- MRS and perfusion MRI to define further the mechanisms of IRIS and identify surrogate markers of PML progression and survival. 2)Analyze the role of IRIS in PML-associated epileptogenesis. We will determine prospectively the value of HCS and of dense array electroencephalographic findings as predictive markers of IRIS and seizures in PML. 3)Decipher the histopathological substrates of PML progression, epileptogenesis and IRIS. We will characterize histologically the pathogenic mechanisms leading to HCS, hyperperfusion and IRIS. Our multifaceted innovative approach will have direct impact on the management of patients with PML-IRIS and seizures. The knowledge gained will greatly advance the fields of Neuroradiology, Immunology and Epileptology.